Self-cleaning systems comprising a layer of photo-catalytic material are known per se. They are used, inter alia, as glazing sheets made of plastics or glass for use as window glass, or are used as, for example, cover plates on buildings as façade design. Especially when used as cover plates on buildings, layers of photo-catalytic material save the time and cost of outside cleaning of the building.
The photo-catalytic self-cleaning activity is caused by the generation of hydroxy and peroxy radicals by the photo-catalytic layer when illuminated with sunlight. The generated radicals oxidize and thus destroy organic dirt on the surface which subsequently may be washed away, for example, by rain. Photo-catalytic self-cleaning coatings constituted of titanium dioxide are well known.
Also for indoor application, the use of a photo-catalytic self-cleaning layer may be beneficial, as is disclosed in US2006/0137708 which provides a process for photo-catalytically treating stains on kitchen tiles and platforms by providing them with a coating of a thin film of photo-catalyst made of a semiconductor such as titanium dioxide uniformly dispersed in dilute silica sol. A light source, such as a fluorescent lamp, is used to irradiate and photo-excite the photo-catalytic thin film by ultraviolet radiation included in the fluorescent light. As a result the stains are decomposed and the layer is auto-cleaned.
A drawback of the known photo-catalytic process is that a relatively high flux of UV radiation is required from the fluorescent lamp.